1. Field of the Invention
This invention relates to optical spectroscopy in general, and to optical spectroscopy systems using concave diffraction gratings in particular.
2. Description of the Prior Art
Optical spectrometry and spectroscopy systems usually operate on one of two principles, refraction or diffraction. A beam of light is either refracted or diffracted to separate the various wavelengths of light from each other so that the presence or absence of light in the respective wavelength ranges or bands can be determined.
Refractive systems typically use some refractive device, usually in the form of a prism, to disperse the beam of light to be investigated into its component wavelengths. Such refractive devices operate on the principle that the refractive index of the device varies with wavelength so that different component colors in a beam of light are angularly dispersed at respectively different angles as they pass-through the refracting device.
Refractive or prism spectrometers have been in use for a long time. However, they suffer from several limitations. For example, the refractive material or medium, usually glass, absorbs certain wavelengths, thereby limiting the spectral range that can be investigated. Refractive materials, such as glass, may also introduce chromatic aberrations, since the focal length of a glass lens is slightly different for each wavelength. Finally, such refractive spectrometers are further limited to relatively small angles of incidence, thus restricting resolution.
Spectrometers can also be made with diffraction gratings to disperse the light by its respective wavelength components. Diffraction gratings are very useful and desirable in spectroscopy because of: their lack of chromatic aberration; their ability to proportionally disperse light by wavelength, even at large angles; and their ability to disperse very wide spectral ranges. The first diffraction gratings were planar, i.e., flat, and were either of the reflective or transmissive type. Reflection gratings usually have a flat, reflective surface with a series of diffractive elements, such as lines inscribed thereon ruled closely enough together to cause diffraction of the wavelengths of interest by means of constructive and destructive interference. Transmission gratings usually have as their diffractive elements either open slits in an opaque surface or transparent rulings or facets on the surface. While reflective and transmissive gratings operate somewhat differently, the results are essentially the same, i.e., both disperse light by wavelength.
While plane gratings are able to disperse light with the advantages listed above, they are unable to focus the dispersed beam. Practical use of a plane diffraction grating, therefore, requires the use of some focusing device, usually a glass lens or concave mirror, to focus the dispersed beam so that it can be studied. Such a glass lens, however, reintroduces the disadvantages of restricted spectral range and chromatic aberration associated with the refractive systems. Concave mirrors, on the other hand, can reduce throughput by the reflection inefficiencies resulting from additional reflections off the concave mirrors. Thus, spectroscopists continued searching for a system that could make full use of the advantages of the diffraction grating without introducing the substantial limitations associated with refractive-type systems. Such a system was found in the concave grating, which is essentially like a plane grating that is reformed to have a curved or concave surface.
An optical spectrometer utilizing a concave diffraction grating was first conceived by Prof. Rowland in 1882. Rowland's spectrometer combined the advantages of the plane diffraction grating with the focusing properties of a concave mirror. The resulting concave diffraction grating not only had the ability to disperse light of wide spectral ranges at large angles of incidence like the plane diffraction grating, but it could also focus at least one component of the dispersed beam without the narrow spectral range limitation and chromatic aberration associated with glass lenses. Specifically, Rowland found that when he ruled the diffraction grating on a blank having the shape of a portion of a spheroidal surface, one component, usually considered to be the vertical component, of the diffracted light fell into focus at a locus of points in space that describe an imaginary circle. This imaginary circle, which has become known as the Rowland circle, has a diameter equal to the radius of curvature of the spherical grating and is tangently located on the grating center. The Rowland circle forms the basis of nearly all vacuum spectrographs in use today.
Spectrometers based on the Rowland circle principal are used for many different purposes such as, for example, determining the wavelengths of light passed through materials to gain knowledge of the characteristics or chemical compositions of such materials or determining the characteristics of the light source. Such spectrometers can also be used as monochromators. A monochromator can be thought of as a precise optical filter. The layout of the monochromator is very similar to that of a conventional spectrometer, except that an exit pupil or slit is sized and positioned in a manner that blocks out all but a specific wavelength of the diffracted beam of light. In this manner, monochromators are useful as sources of monochromatic light to study the effects that one wavelength of light has on other elements. An example would be the use of a monochromator for emission spectroscopy where the monochromatic light output by the monochromator is passed through a gaseous sample to excite the various atoms therein. The emitted light from the excited atoms is then analyzed by passing it through yet another spectrometer. Emission spectroscopy often requires a fairly energetic monochromatic light source to properly excite a sample so that it emits sufficient light. Because of this high energy requirement, monochromators are sought that have as much light output, or throughput, as possible.
Quite often, these above listed spectroscopy applications are performed in a high vacuum environment to prevent the absorption of ultra-violet and higher frequencies of light by air molecules. Such spectrometers are usually called vacuum spectrographs or vacuum monochromators because the gratings and optical path are contained within vacuum chambers.
While the Rowland circle concept certainly represented such a substantial breakthrough in diffraction spectroscopy that it is still the standard in use today, it still suffers the significant drawback of introducing substantial astigmatism in the diffracted image. That is, only that component of light at right angles to the grating rulings, usually called the vertical component, is focused on the Rowland circle. The component of light that is parallel to the lines of the grating, usually called the horizontal component, falls into focus at some different point in space that is off the Rowland circle with one exception. The one exception is that stigmatic focus occurs theoretically at one point on the Rowland circle where a line that is normal to the center point of the grating intersects the Rowland circle, which also coincides with the center of curvature of the spherical grating. However, it is a physical impossibility to utilize this stigmatic imaging point fully, because such stigmation could only occur if the light source or entrance slit and the exit aperture or detector occupy the same position on that stigmatic point on the Rowland circle, which is impossible. Also, since that point is on the optical axis of the grating, no diffraction occurs. Usually, the horizontal component of the diffracted light focuses at a point that is a greater distance from the grating than the Rowland circle.
This astigmatism on the Rowland circle, i.e., focused vertical component but unfocused horizontal component, causes a substantial loss in the intensity of the diffracted beam as the resolved light in each wavelength or band is stretched out in a protracted line image instead of being concentrated at a point. This loss of intensity and stretching out of the wavelength band results in a total loss of any information relating the diffracted image to specific height positions along the entrance slit. Thus, imaging 10 spectroscopy, where simultaneous spectroscopy of two sources or a complex image falling on the entrance slit, is impossible with an uncorrected diffractive spectrometer. The loss of intensity also reduces the available light or throughput in monochromator applications, thereby limiting usefulness. Moreover, the increased resolving power of newer gratings requires the use of narrower lines and finer grained plates, which further accentuates the intensity loss problem.
While the Rowland circle type mountings do have the disadvantage of substantial astigmatism, as described above, they have such excellent focus resolution and sharpness in the vertical component that the disadvantages of the astigmatism have generally been tolerated for most spectroscopic applications for decades. However, if the astigmatism could be eliminated, detection efficiency, throughput, and signal-to-noise ratio could be improved, because both the vertical and horizontal components would be focused together to converge the diffracted image to a focused, high luminosity point instead of the protracted line. Also, comparative spectroscopy, where two sources are projected through two halves of the same entrance slit, would also be enhanced. Such stigmatic imaging could also allow for two-dimensional spectroscopy, that is, where spectral and slit position information are gathered simultaneously. Thus, a person or electronic detector equipment could identify the light spectrum or wavelength bands present or absent at each point along an elongated entrance slit, instead of having to use a point entrance or source. Such an elongated entrance slit could, for example, be wiped or moved across an image of the sun or across a candle flame or other non-point light source, while taking continuous or intermittent real-time measurements of wavelengths at selected points along the entrance slit or even continuously along the entrance slit to obtain a two-dimensional reading of all the wavelengths present at all points on the sun or candle flame image. Finally, if the primary aberration of astigmatism could be eliminated, correction of other minor aberrations could be made more easily.
Modern spectroscopy developments have further highlighted the limitations caused by the astigmatism. For example, investigation of the hyperfine structure of spectral lines by either Fabry-Perot interferometry or Lummer plate interference patterns is not possible with an instrument that does not yield a stigmatic image of the entrance slit. For these applications, prism spectrographs are still used, which, of course, trades away the advantages inherent with the diffraction type spectrographs discussed above for such spectral line investigations.
Besides the technique of trying to mount an inlet slit or source and an outlet aperture or detector very close to each other adjacent the one theoretical stigmatic image point on the Rowland circle that is coincident with the center of curvature of the spherical grating, which is impractical and severely limiting, as described above, there have been, prior to this invention, only two options available to get stigmatic or near stigmatic imaging. The first of these previously known methods to achieve stigmatic or near stigmatic imaging has been to reshape the spherical diffraction grating or blank, and the second method is to change the densities, configurations, and orientations of the rulings. By varying the shape of the grating or blank to something other than a pure spherical surface, for example by closing the radius of the grating blank in the vertical dimension, the focal length of the horizontal component can be diminished while leaving the focal length of the vertical component the same, thereby collapsing the normal line image from a spherical grating down to a point. Such aspherical surfaces, usually ellipsoidal and toric, have been made and ruled normally. However, they are much more expensive and difficult to make than spherical gratings, and mathematically they are limited to being stigmatic at only one or two places on the Rowland circle, which is also quite limiting in utility.
In the second alternative, the spherical surfaces have been ruled non-normally, i.e., with grating lines variably spaced to vary the ruling density, or with grating lines that are curved or tilted to follow customized corrective specifications. These hybrid spherical gratings with modified ruling or grating lines are very difficult to make, very expensive, and they still have stigmatic points in usually only one or two, and so far no more than three places on the Rowland circle. There have been recent claims that holographical gratings with varied rulings have been made that can produce a line of focal points out of the Rowland circle. However, in addition to the difficulty and cost of manufacture, various performance characteristics of such alternate grating blanks, such as quality of rulings, and intensity characteristics, are almost always inferior to the normally ruled, spherical grating blanks known in the industry as the Type I devices.
Until this invention, spectroscopists had generally concluded that astigmatism, since it is present in all off-axis uses of spherical reflecting surfaces, must also be inescapable with concave gratings. Consequently, mountings developed for concave diffraction gratings have, with very few exceptions, been based on the Rowland circle concept, because the points on the Rowland circle have always provided unsurpassed spectral resolution, albeit astigmatic. Spectroscopists have generally accepted the severe astigmatism as the price to be paid for the high resolution, high quality spectral range, and affordability of the Type I spherical gratings.